Knives with automatic blade retraction permit automatic displacement of the blade from the extended position in which the blade projects at least portion-wise out of the housing of the knife into the retracted position in which the blade is usually disposed completely in the housing of the knife, irrespective of the actuating state of the actuating device.
The movement of the blade can thus be decoupled from the movement of the actuating device, by means of which the blade can usually be pushed out of the housing with the aid of the thumb or the index finger. Decoupling of the movements of blade and actuating device is implemented by the action of a cutting force on the blade. The action of the cutting force leads to a decoupling movement of the blade which can be for example in the form of a rotary movement. In the decoupling situation the actuating device releases the blade so that the blade and the actuating device can move independently of each other.
After the action of the cutting force on the blade is removed after the termination of the cutting operation the blade is moved directly and independently of the state of the actuating device from the extended position into the retracted position by means of the retraction element.
Knives with automatic blade retraction are designed in particular for situations which involve unintentional movements during the cutting operation. Frequently the cutting capability of the blade or the resistance that the material to be cut can put up is incorrectly estimated by users of the knives of that kind so that the user of the knife, by virtue of an excessively high application of force, loses control over the movements involved as soon as the blade leaves the material being cut. By virtue of those uncontrolled movements of the knife a knife with the extended blade would represent a serious risk of injury to the user of the knife himself, and also for people in the proximity. That can also involve unintentional damage to articles around it.
Such injuries to people and damage to articles due to unintentional and uncontrolled movement of the knife are avoided by automatic blade retraction after the action of the cutting force on the blade ceases. By virtue of the enhanced safety aspect during use of such knives they are also referred to as safety knives.
The safety knives in the state of the art generally involve a complex mechanism which, in the event of excessive stressing, are exposed to a high level of risk of a malfunction. In particular the blade guidance and the decoupling mechanism of known safety knifes regularly suffer malfunctions if the knife housing is damaged or deformed by virtue of being overstressed.
A safety knife with automatic blade retraction is known for example from EP 1 864 766 B1. The proposed safety knife has a blade unit which is rotated in the housing under the effect of a cutting force and thus causes decoupling from the manually actuable actuating device. Because of the limited structural space within the knife housing longitudinal guidance for the blade unit is embodied by a pin which is arranged laterally on the blade unit and which is displaceable in a slot in the knife housing.
In the case of longitudinal guides of the above-described kind it can happen that, in the event of the knife being overstressed, for example due to an excessive transverse force acting on the knife housing, the guide elements can come out of engagement by virtue of the housing flexing. That would seriously adversely affect the functionality of the knife.
A safety knife is also known from DE 10 2011 007 234 B3 which has recourse to a separate coupling element in order to implement coupling or decoupling of the manually actuable actuating device and the cutting blade. In the proposed safety knife the mechanism is expanded by an additional delicate coupling mechanism.